Telephone system



Nov. 8, 1938. C. E. LOMAX TELEPHONE SYSTEM Filed Dec. 6, 1937 9 Sheets-Sheet 1 Bom N .GI

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CLARENCE E. LOMAX Mm VM Nov. 8, 1938'. c, E, LolMAx TELEPHONE SYSTEM Filed Dec. 6, 1937 9 Sheets-Sheet 2 Nov. 8, 1938. c, E. LoMAx 2,135,822

TELEPHONE SYSTEM I Filed-Dec. e, 1937 Q'SnQetS-sneet s INVENTQR. CLARENCE E. I OMAX BYS "1a. l l

ATTORNEYS.

Nov. 8, 1938.- c. E. LoMAx TELEPHONE SYSTEM Filed Dec. 6, 1957 9 Sheets-Sheet 4 Nov. 8, 1938. c. E. LoMAx 2,135,822

I TELEPHONE SYSTEM Filed Dec. e, 1937 9 sheets-sheet 5 INVENTOR.

CLARENCE E. LOMAX ATTORNEYS Nov. 8, 1938. c. E. LoMAx TELEPHONE SYSTEM Filed Dec. 6, 1937 9 Sheets-Sheetl 6 Nov. 8, 1938. c. E. LoMAx 2,135,822

TELEPHONE SYSTEM Filed Dec. e, 1937 9 sheets-sheet 'I Emu .www

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Nov. 8, 1938. c. E. LoMAx TELEPHONE SYSTEM 9 Sheets-Sheet 8 Filed Dec.. 6, 1937 em). Tlu Pimm@ INVENTOR.

CLARENCE E. LoMAx Nav. 8, 1938. c. E. LoMAx TELEPHONE SYSTEM Filed Dec. 6, 1957 9 Sheets-Sheet 9 mbombo-Q Inl.. NFO- ONS C. m22 95 g Quo.

INVENTOR.

CLARENCE E. LOMAX BY a i z s. V.. m n A Patented Nov. 8, 1938 PATENT OFFICEI TELEPHONE- SYSTEM Clarence E. Lomax, Oak Park, Ill.. assigner to Associated Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Application December 6, 1937, Serial No. 178,276

37 claims.

The present invention relates to telephone systems and more particularly to power, switching and supervisory apparatus for use in small telephone exchanges of the automatic type. l

The operatingadvantages of the automatic telephone exchange, as contrasted with the manual type of exchange, are well known in the art. Because of these advantages, the present trend in the installation of telephone systems favors the use of automatic equipment in any replacement or original installation irrespective of the size of the system. In the construction of exceedingly small exchanges for use in areas of limited population and usually designed to serve one hundred subscribers lines or less, the problem of maintaining the manufacturing and installation costs below a prohibitive value without sacrificing reliability of operation is a very real one. This is particularly true in small exchanges which are entirely automatic in character, as contrasted with the well-known type of remotely controlled automatic exchange, for the reason that the auxiliary equipment included therein, namely, the power and supervisory apparatus, must necessarily be as complete and extensive as the corresponding apparatus used in larger installations. In the past, little attention has been directed to the possibilities of reducing the construction cost of the small automatic exchange through the expedient of simplifying the power and supervisory apparatus which forms a necessary adjunct of the line switching apparatus per se.J On the contrary, it has been the practice to adopt more or less in toto the form of this apparatus as used in the larger exchanges, only minor alterations being made therein for the purpose of adapting the apparatus to the `extensive operating requirements inherently present in the small automatic exchange. More specifically, it has been the prior practice to provide a separate and distinct unit for each individual supervisory function; i. e., separate units are provided for: generating coded ringing current for the selective energization of signalling devices associated with a called line of the party type; supervising the allotter means conventionally provided for switch link assignment purposes; supervising the line lock-out and link release apparatus usually embodied in each switch link; and other purposes. As a result, the element of cost represented by the power and supervisory apparatus is disproportionately large in prior exchanges of small size.

It is an object of the present invention, therey fore, to provide in an automatic telephone system, improved and exceedingly simple apparatus for performing, in a thoroughly reliable manner, any and all of the functions ordinarily required in a small telephone exchange.

More specifically, it is an object of the inven- 5 tion to provide, in a. telephone system of thecharacter briefly referred to above, power and supervisory apparatus so arranged that the reqvuislte functions of the apparatus are consolidated in a minimum number of circuit elements. lo

The invention as described in detail hereinafter is illustrated in its embodiment in a small community automatic exchange designed to serve a plurality of subscribers lines of the party line type and a group of two-way trunk lines 15 interconnecting the community exchange with a main or central exchange of the manual type. Finder-connector links employing rotary switches are used exclusively for the line switching functions, allotter means being provided, in accordance with well-known practice, for assigning the links to successive calling lines in a deflnite order. Impulse responsive means are provided in the connector portion of each link through the operation of which the associated connector 5 switch is caused to perform its line selecting functions in response to impulses transmitted thereto by way of a calling subscribers line from a calling substation. Inconformance with the use of party lines, namely, lines having more 30' than one substation connected thereto, the power and supervisory apparatus is arranged for selective ringing over either side `of a called line. Specifically, this apparatus includes a source ofv alternatingringing current, a plurality of ringing current conductors and means comprising a rotary interrupting switch for intermittently connecting the conductors to the source so that each thereof has impressed thereon the voltage of the source for coded intervals which'are different for each of the lines,

In general, the objects as set forth above are in part attained in accordance with the present invention by assigning twoor more functions to at least a portion of the elements involved in 4- generating the interrupted ringing current. Thus, means controlled by the interrupter switch are provided for assigning a second linkv to search for a calling line in the event a first assigned link fails to nd the said calling line; 50 means controlled by the interrupter switch are provided forreleasing a link seized by oneof the lines and for preventing another link from being l assigned to the line in the event the connector portion of the seized link is not caused to select 55 a called line within a predetermined time interval after the link is seized; and means controlled by the control means for the switch are provided for causing the connector portion of a seized link to search for an idle one of the trunk lines on a call which is to be extended by way of one of the trunk lines. In accordance with a further feature of the invention, an improved arrangement is provided for applying, without interference, ringing current to a calling line in the course of completing a desired connection on a reverting call, i. e., a call in which a subscriber having a telephone connected to one subscribers line desires a connection with a telephone connected to the same line. Further features of the invention pertain to the arrangement for checking the condition of the apparatus in the community exchange from a remote point, the arrangement for indicating an all-trunks-busy condition on a trunk call to the distant manual exchange, and the particular arrangement of the circuit elements whereby the apparatus required is reduced to an absolute minimum.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The `invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in -which Figure 1 illustrates diagrammatically an automatic telephone system of thetype to which the present invention is applicable; Fig. 2 illustrates the figure layout for the remaining figures of the drawings; Figs. 3, 4 and 5 illustrate the details of one of the finder-connector links incorporated in the system of Fig. 1; Fig. 6 illustrates the allotter means and a portion of the power and supervisory apparatus embodied in the system; Fig. 7 illustrates the remaining portion of the power and supervisory apparatus; Figs. 8 and 9 illustrate a two-way repeater terminating one of the trunk lines of the system; and Fig. 10 illustrates the apparatus terminating the opposite cnd of the trunk line extending to the repeater illustrated in Figs. 8 and 9.

Referring now more particularly to Fig. l of the drawings, the system there shown comprises a small community exchange I0 and a manual exchange I2 interconnected by a group of trunk lines, one of which is indicated in the drawings as terminating at a two-way repeater circuit 20 forming a part of the apparatus located at the exchange I0. Also extending to the exchange I0 are a plurality of subscribers lines, two of which are indicated at II and I6 having connected respectively thereto the subscribers substations A and B. The subscribers lines may be either of the two-conductor type or of the well-known grounded type employing but a single conductor, or both. With the arrangement described in detail hereinafter, all grounded lines are equipped with adapters in order to permit the desired switching and signalling functions to be successfully carried out. Although each of the two lines shown is illustrated as having only one telephone substation connected thereto, it is contemplated that a plurality of substations will be connected to each line or, in other words, that each subscribers line will be of the party type. Each of the substations connected to the various subscribers lines may comprise the usual hand set having embodied therein a dial controlled impulsing device of conventional arrangement.

rendered accessible to the finder portions of the y finder-connector links included in the exchange, and by way of which it is terminated in the banks of the connector portions of the various links. Thus, the line II terminates at the line circuit 2l through which it is connected to the bank contacts of a connector switch 22 forming a part of the finder-connector link 22 and by way oi' which it is rendered accessible to the finder portion 24 of the link 23. Similarly, the line Il terminates at the line circuit 25 and the illustrated two-way trunk interconnecting the exchanges III and I2 terminates at the repeater 20 vconnected to the line circuit 281.

The exchange In is designed to serve only a small number of subscribers lines. specifically nineteen, and a small group of three trunk lines and, accordingly, all of the subscribers lines and trunk lines are arranged to be served by any one of the plurality of finder-connector links included therein. Obviously, the number of links required for satisfactory seryice will depend upon the volume of telephone traiiic, but, taking into account the character of the exchange. four links are considered adequate for reasonably good service. In order to assign the links to successive calling lines in a definite order so that a link is always available for use by a calling line, excepting, oi' course, those instances when all of the links are simultaneously busy, there is provided allotter 'means as indicated at 2`l. This allotter means is suitably` interconnected with each of the links, each line circuit, and also with the power and supervisory apparatus indicated at 28. The power and supervisory apparatus is interconnected with each of the links in the manner indicated.

Neglecting for the present the details of the circuit arrangement, the system as briey described above is in general quite similar to conventional systems, the operation of which is well understood in the art. In brief, when a subscriber, such, for example, as the subscriber at the telephone A, initiates a call over the line Il to a second subscribers telephone, such, for example, as the telephone B, by lifting his receiver from its hook, the line circuit 2| causes the finder-connector link previously assigned by the allotter means 21 to the calling line to be started in operation to search for and seize the calling line. Assuming that the link 22 is delegated by the allotter means 21 to set up the connection, the finder portion 24 of the link operates to select the calling line and to connect it through to the conductors of the link line incoming to the connector portion of the link. When the calling line I I is seized and extended by way of the finder switch to the associated connector switch, the control equipment of the last-mentioned switch is automatically conditioned to be responsive to the rst series of impulses transmitted from the impulsing device at the substation A.

In response to the dialing of the first digit at the calling substation, the connector switch 22 is caused to operate in a novel manner to select the group of lines including the called line Il. Thereafter, and in response to the dialing of the second digit, the called line IBis selected from the selected group of lines. When the called line is selected, the control apparatus included in the link circuit 23 functions to test the condition' of this line, thereby to determine the busy or idle condition thereof.

If the called line is found busy, busy-tone current generated by a portion of the power and supervisory apparatus 28 is applied to the loop extending to the calling subscribers substation, thereby to indicate to the calling party that the desired connection can not immediately be completed. If, on the other hand, the called line is idle at the time it is selected, the calling subscriber may proceed to dial the third and iinal digit for the purpose of selecting ringing current of the particular code which corresponds to the called substation B connected to the line I8. At the conclusion of this digit, the control apparatus embodied in the link circuit 23 functions to apply the selected coded ringing current to a selected side of the line I6, thereby to energize the signalling device at the substation B in accordance with the code identifying this substation. During the ringing operation, ring-back tone current is applied to the loop extending to the calling subscribers substation to indicate to the calling party that the desired substation is being signalled. Finally a connection between the calling and called lines is completed when the call is answered at the substation B. As pointed out with particularity hereinafter, the arrangement of the link circuit is such that a link utilized in setting up a desired connection is released only when the connection is cleared out at the calling substantion.

The switching apparatus located in the exchange I is also utilized in setting up a connection between two substations connected to the same line, the sole function of the apparatus in this regard being that of selecting and applying ringing current to the calling line of the proper code to signal the called party. The discharge of this function necessitates the seizure of the calling line by an idle link and the operation of the connector portion of the link to select the calling line, all in the manner outlined above. Thereafter, and following the dialing of the ringing current code selecting digit at the calling substation, the calling party restores his receiver to its hook and ringing current of the selected code identifying the called substation is projected over the calling line. When the call is answered, the operated link is released and the line circuit of the line in service operates to busy the line and thereby prevent its seizure by the connector portion of another link on a subsequent call. Finally the line circuit of the line is restored to normal when the connection is cleared out.

In the operation of the link to select one of the small group of trunk lines outgoing from the exchange I0 and extending to the exchange I2, only two predetermined digits are required to cause the connector portion of the link to select the group of trunk lines. At the conclusion of these two digits and, in accordance with one feature of the present invention, certain of the elements embodied in the link circuit 23 cooperate with a portion of the equipment in the power and supervisory apparatus 28 to cause the connector switch 22 to operate in the manner of a trunkhunting selector switch. Immediately the series of impulses comprising the second digit is concluded, this last-mentioned apparatus is started into operation to cause the connector switch 22 to search for an idle one of the selected group of trunk lines. When an idle trunk line is found, the trunk-hunting operation is interrupted and the calling line is extended by way of the selected trunk line and the associated two-way repeater circuit `to the terminating apparatus located at the distant exchange I2. In this connection, it is noted that two sets of link line conductors are provided in the link circuit 23, one of which is devoid of all voice frequency coupling elements, such, for example, as condensers, and is utilized in extending a desired connection by way of one of the trunk lines. Following the extension of the connection to the trunk line terminating apparatus located in the exchange I2, the connection may further be extended to the desired called line through the intervention of an operator in the manner conventionally followed in manual exchange operation. Following the establishment of a connection in the above-described manner, the release of the connection is entirely under the control of the operator in the exchange I2.

Since the trunk lines interconnecting the exchanges I0 and I2 are of the two-way type, it follows that any one of these lines and the switching apparatus included in the exchange I0 may be utilized in the completion of calls originating or extending by way of the exchange I2 and which are to be extended to one of the subscribers lines terminating at the exchange I0. The mode of operation of the equipment in the exchange I0, following the extension of a call thereto by way of one of the trunk lines noted, is exactly the same as that set forth above with reference to the connection established between the substations A and B, it being observed that each of these trunk lines terminates in a line circuit identical with that associated with each of the subscribers lines. Here again the release of the connection is entirely under the control of the intervening operator who originates the call in the exchange I2.

Considering now in greater detail the apparatus embodied in the exchange I0 and involved in the above-described sequence of operations, and referring more particularly to the apparatus illustrated in Figs. 3, 4 and 5, there are illustrated the elements of the line circuit 2I terminating the subscribers line I I, and the finder-connector link 23. The line circuit comprises a line relay R3I0,

a cut-off relay R320, a'nd a line lock-out relay R300, all arranged in accordance with conventional practice. This circuit also includes a ccndenser 305 through which pay-station tone current may be applied to the control conductor C394 of the line II from an alternating current source, not shown, for purposes referred to specifically hereinafter. This pay-station tone circuit is shown in association with the line II for illustrative purposes only, it being pointed out that a circuit of this form is provided only in association with those of the lines terminating at the exchange I0 having telephones of the paystation type connected thereto. Since the apparatus for applying pay-station tone current to the conductor C394 may be entirely conventional in character, an illustration thereof is omitted.

The line circuits respectively associated with each of the other lines extending to the exchange I0 are identical with that terminating the line II, and, hence, that terminating the trunk line |00 and indicated at 381 is only diagrammatically illustrated.

The finder portion of the link 23 comprises the stepping switch 380 and the line switching relay R350. The switch 380 is of the standard rotary type and comprises three sets of bank contacts 384, 385, and 386, of twenty-five points each, wipers 38|, 382 and 383 respectively associated with the contact sets 384, 385 and 386, and means comprising the motor magnet M330 and a ratchet and pawl mechanism, not shown, for driving the enumerated wipers over the contacts of their respective associated sets. The connector portion of the link similarly includes a rotary switch 550 of standard 'construction which comprises bank contact sets 55|, 552, 553, 554 and 555, Wipers 58|, 582, 583, 584 and 585 respectively associated with the contact sets in the order of their enumeration, and means comprising the motor magnet M540 and a ratchet and pawl mechanism, not shown, for driving the wipers over the contacts of their respective associated contact sets. As regards the connector portion of the link, the conductors of the lines extending to the exchange are terminated at the contacts of the sets 55| and 552 of the switch 550 and the private or control conductors of these lines extend to the contacts of the set 553. Specifically, the subscribers lines are divided into two groups comprising ten lines and nine lines, respectively. 'I'he lines of the ten-line group are connected to the contacts numbered from three to twelve of the indicated contact sets while the lines of the nineline group are connected to the contacts numbered from fourteen to twenty-two. The contacts corresponding to the twenty-third, twentyfourth, and twenty-fifth off-normal positions of the switch are reserved for terminating the conductors of the three trunk lines interconnecting the exchange I0 with the exchange i2.

Also included in the link circuit is a ringing current code selector switch 560. This switch, which is of the well-known minor type, includes two sets of bank contacts 56| and 562, wipers 563 and 564 respectively associated with the contact sets 56| and 562, means comprising thc motor magnet M565 and a ratchet and pawl mechanism, not shown, for driving the enumerated wipers over their respective associated contacts, a release magnet M566 operative to restore the switch to normal, and a set of off-normal springs v510 for controlling certain of the relays referred to hereinafter. The switch 560 has terminating in its upper set of bank contacts, in the manner indicated, five ringing current conductors C|54 to C|58, inclusive, which lead to the power and supervisory apparatus illustrated in Fig. 7 by way of the cable |50.

Although the particular arrangement of the control relays will best be understood by reference to the detailed description of the operation which follows, it is believed that a brief reference to the functions performed by certain combinations of these relays will facilitate an understanding of the features of the circuit to which the present invention relates. In this connection, it is pointed out that R310 is the usual line relay which responds to impulses transmitted over a calling line to the link circuit; R403 is the usual hold relay for holding the nder portion of the link operated following the seizure thereof and during impulsing; R4|0 is a relay which operates to repeat impulses transmitted to R310; R420 is a slave relay controlled by R403; and that these four relays, in combination with the circuits controlled thereby, comprise impulse responsive means for causing the connector switch 550 to select any desired subscribers line or the group of trunk lines terminating at the exchange and for causing the code selector switch 560 to select a desired ringing current conductor for connection to the selected line. Transfer of the impulsing circuit from the magnet M540 of the switch 550 to the magnet M565 of the code selector switch 560 is a function assigned to R490, which relay operates at the end of the second digit dialed at a calling substation and is directly controlled by R460 and R450. Testing of a called line selected by the connector portion ot the link to determine its idle or busy condition is achieved through the provision of the busy relay R410, the operating circuit for' this relay being held prepared during second digit impulsing by means comprising the relay R450. The application of ringing current to a selected called line is an assigned function of the ringing relay R530, this relay in turn being controlled by the interrelated operation of R400' and the coue selector switch 560. As pointed out in detail hereinafter. the selection of a particular ringing code and the proper application of ringing current to a selected called line is tantamount to the selection of one of the group of ringing current conductors C|54 to CI50, inclusive, and the connection of this selected conductor to project ringing current over only one side of the selected line, ground return ringing being utilized throughout.

The interruption of the ringing operation when a called substation on a selected called line answers is accomplished in the usual manner through the operation of R520, which relay will be recognized as the conventional ring cut-off relay. The operation of this relay also results in the completion of a talking loop to the called subscribers substation and in the completion of a circuit including this loop for energizing the supervisory relay R400. The last-mentioned relay is a slave relay which controls the energization of the battery reversing relay R360. This battery reversing relay performs the usual function of reversing the polarity of the direct potential applied to the loop extending to a calling subscribers substation for metering or other supervisory purposes.

In order to adapt the link circuit for efficient ringing during the establishing of a reverting call and for additional purposes pointed out hereinafter, there are provided two additional relays R430 and R350. The first of these relays is energized only in the course of completing a reverting call and functions to control R350, which latter relay is a switching relayA eiective to switch theline conductors of the calling line from a connection with the set of link line conductors C|04 and C|00 to a connection with the set of link line conductors C|05 and C|08. 'I'he purpose of thus switching the conductors of a calling line to C|05 and C|09 on a reverting call is to remove the winding of R400 from across the ringing circuit thereby to prevent this winding from shunting a substantial portion of the linging current during the ringing operation.

A further function of R350 is that of switching the conductors of a calling line, in the manner noted, in the course of completing a connection extending by way of one of the trunk lines interconnecting the exchange I0 and the exchange, |2 so that the voice coupling condensers 308 and 390 are removed from the loop circuit. In this case the operation of the switching relay R350 is under the control of R500 which latter relay also enters into the trunk-hunting operation of the rotary switch 550 during the movement of the switch to select an idle outgoing trunk line.

As pointed out in the introductory portion of the speciication, one feature of the invention is concerned with the provision of means for locking out a calling line in the event a connection is not established to a desired called line within a predetermined time interval after the seizure 1I of one of the links. Specifically, this means comprises. in addition to portions of the power and supervisory apparatus illustrated in Fig. '1, the relay R440' and the line and lockout relays, such, for example, as the relays R320 and R300, embodied in the line circuit 21.

In order to adapt the link circuit illustrated for use in connection with lines of the grounded type, i. e., lines including a single line conductor and a ground return path, there is'provided, in

addition to the apparatus already mentioned, the-l relay R510 which functions to control certain of the relays in the link circuit only in the event a line of the grounded type is selected.

In order to remove the finder-connector link from service for purposes of repair or testing, there is included therein the manually operable switch 395. This switch is normally maintained in the position shown in the drawings but, -when operated to its off-normal position, functions in a conventional manner to mark the link as busy in the allotter means, thereby to prevent the link from being assigned for use. f

Referring now more particularly to the allotter means as illustrated in Fig. 6, this means comprises, in brief,l a rotary switch 610, a link start relay R610, a hold relay R605, an impulsing relay R620, a link transfer relay R625, and an alllinks-busy relay R600. The switch 610 is of the standard rotary type and includes four sets of bank contacts 616 to 618. inclusive, wipers 611 to 614 respectively associated with the bank contact sets in the order of their enumeration and means comprising a ratchet and pawl mechanism, not shown, and a motor magnet M660 for driving the wipers step by step over the contacts of their respective associated contact sets.

As indicated above, this allotter means is arranged to control the selection of an idle link from a group of four links, the various control conductors being terminated in the bank contacts of the switch 610 in the manner illustrated. As will be apparent from the detailed description which follows, the mode of operation of the allotter means ismore or less conventional in form as regards its function toselect idle ones of the associated links. It is noted, however, that the manner in which the link transfer relay R625 cooperates with certain of the elements embodied in the power and supervisory apparatus, to assign a second link to a calling line upon failure of a first assigned link to find the line, departs from conventional practice and forms the subject matter of one feature of the present inven tion.

"The power and supervisory apparatus which is illustrated in the lower portion of Fig. 6 and in Fig. 'l comprises, in brief, a source of ringing current in the form of a mechanical inverter shown in the lower left-hand portion of Fig. 6, the five ringing current conductors C154 to C158, inclusive, and means comprising the rotary switch 110 for intermittently connecting these conductors to ringing current source so. that each thereof has impressed thereon the voltage of this source for coded intervals which are diierent for each of the conductors. The last-mentioned means also comprises the control means for the switch 11, which control means specifically incudes the relays R101, R110, R115 and R120. Also arranged to be controlled byv the four relays noted are the self-restoring relays R100 and R105 which comprise respectively a source of busy-tone current and a source of dial tone current. lThe four relays forming the control means for the switch 110 also function to control the two relays R650 and R655, which latter relays, in combination with R500 or a correspondingrelay embodied in another' of the links', ,comprise means for causing the connector portion of a seized link to select an idle trunk outgoing from the exchange on a call which is to be completed by way of one of these trunks.

The switch -110 not only functions as a ringing current interrupter switch but, in addition, forms an integral part of a 'timing unit for applying supervisory groun pulses to the three supervisory conductors C131, C135 and C146. 'I'his unit includes. in addition to the switch noted, the four cyclically operating relays R124, R130, R135 and R140. 'I'he arrangement is such that the ringing apparatus is utilized for alarm purposes for indicating to a supervisory substation the condition of the equipment in the exchange 10. To this end the three relays R150, R155 and R160 are embodied in the power and supervisory apparatus, which relays control, in accordance with the condition of the equipment, the characteristic of the voltage applied to the alarm conductor C152 and utilized for checking the condition of the equipment from a remote point.

The two-way repeater illustrated in Figs. 8 and 9 associated with one of the trunklines interconnecting the exchanges 10 and 12 and the apparatus terminating this trunk line in the exchange I2, as illustrated in Fig. 10, are of more or less conventional arrangement. Since this portion of the system is illustrated primarily for the purpose of insuring a complete disclosure with reference to the operation of the equipment located in the exchange 10, it is thought that a detailed consideration of the various elements included therein is unnecessary at this point in the description, the function of these elements being pointed out with particularity in the following description. All of the relays included in this equipment, as well as the relays specifically mentioned above, are illustrated in accordance with well-dened conventions to indicate the particular operating characteristics possessed thereby.

Operation of the finder switch. to seize the calling line Referring now more particularly to the operation of the apparatus located in the rural oilice 10 to establish a connection between the lines 11 and 16 on a call from the subomce A to the substation B and assuming that the link illustrated in Figs. 3 to 5, inclusive, is the link delegated by the allotter means shown in Fig. 6 to set up the connection, the operation of this link is initiated when the calling party atv the substation A removes his receiver from its hook to complete the usual subscriber's loop circuit for energizing the line relay R310. The circuit noted extends from ground at RA322 by way of the lower winding of R310; C393, the directcurrent bridge across C393 and C392 at the substation A, C392, RA321 and Ilthrough the upper winding of R310 to battery.

contact set 000 which corresponds to the calling line. At RAOII, the re1ay'ROI0 applies ground to the start conductor CI24 to start the ilnder portion oi' the link in operation to search for and seize the calling line.

Thus. when ground is applied to CI24, a circuit is completed for energizing RlIl in series with the multiply connected motor magnets M000 and M040. In this connection it is pointed out that, with any one of the several links embodied in the oilice I0 idle, R000 is in an energized condition so that the above-noted circuit for ROIl is coinpleted immediately upon the operation oi' the line relay ROI0 or any other line relay. This circuit extends from ground on CI24 by way of RA002 and the winding of R0 Il where it divides, one branch extending by way o! RAO0I, MAO0I and the winding of M000 to battery, and the other branch extending by way oi' RAOI2, CI00, the switch wiper 01|, CI00, RA040, RC040, the switch springs 000 and 001, CI2l, RA420, CIIl, RA000. CI2I, RA400, RA40'I', RC400', Clll, MA042 and through the winding oi.' M040 to battery. Due to the resistance of the winding of ROI0, the two magnets noted, namely, M000 and M040, are not suiiiciently energized t attract their associated armatures and, hence, remain inactive. The relay ROI0, however, operates to interrupt. at RAOI2, a point in the above-traced branch of the circuit described which extends to M040. At RAOI0 the relay ROI0 prepares a circuit. traced hereinafter, ior energizing R020 and, at its amature RAOI0, it completes a circuit extending by way of RCOI0 for energizing R000. The operation of R0|0 also results in the energization of the motor magnet M000 embodied in the rotary switch 000 over a circuit extending from ground at RAOI0 by way oi RCOI0, RA02I, CIOI, the switch wiper 014, Ci20, RA040 and through the winding of M000 to battery. At RAOI'I, the relay ROI0 applies multiple ground to CI20, thereby to maintain R000 operated independently oi' the condition of the other links embodied in the oice I0. At RAOI I, the relay ROI0 completes a circuit for energizing the upper winding of R010, this circuit extending from ground at RAOII by way of the resistor 00|, Ci04, the switch wiper 012, CI20 and through the upper winding of R010 to battery.

The relay R010 operates when its upper winding is energized, and completes, at RA01I. a circuit for energizing R4I0, this circuit extending from ground at RA401 by way of RC400, CIIO, RAJII, CI Ii and through the winding of R4I0 to battery. At RA012, the relay R010 prepares a holding circuit for R400. The operating circuit for the last-mentioned relay is completed upon the operation ot R4I0 and extends from ground at RA4I2 by way of RC4I0, CIIO, and the winding of R400 to battery. The relay R4I0 upon operating also completes a holding circuit for itself, this circuit extending i'rom ground at RA4II by way of the resistor 440, CIIO, RAlH, CIII and the Winding of R4I0 to battery.

'Ihe relay R403, upon operating, similarly completes, at RA401, the above-noted holding circuit for itself, which circuit extends from ground at RA401 by way of RC400, CIIO, RA012, RCO'I0, CIIS and through the winding of R400 to battery. The relay R400 further completes, at RA405 and RA400, a circuit for energizing R420, this circuit extending from ground at RA442 by way of RA400, RA405, and the winding of R420 to battery. At RA400, the relay R400 also prepares a holding circuit, traced hereinafter. for

the switching relay R040.

The relay R420 upon operating prepares, at its armatures RA42| and RA420, Vholding circuits for certain of the relays specifically referred to hereinafter. When R420 operates it also removes, at RA42I, ground from the conductor CI20 thereby to open one oi' the multiple holding circuits for R000 embodied in the allotter. The relay R420, upon operating, further completes, at RA421, a circuit for energizing the relay R120 embodied In the power and supervisory apparatus illustrated in Fig. 7, this circuit extending from ground at RA42'I by way oi the start conductor Ci22 and through the winding oi' R120 to When R000 operates substantially simultaneously with the operation oi' R010, it prepares at RA000 a circuit,'traced hereinafter, for energizing M000 and, at RAlll, completes an obvious multiple holding circuit for R000. At RAlll, the relay Rlllextends ground through the upper winding of R020 and the switch wiper 010 to the test conductor CI21, thereby to provide a path over which the upper winding of the switching relay R040 may be energized when the wipers oi' the rotary switch 000 are subsequently stepped to a position corresponding to the calling line Il The magnet M000, upon being energized in response to the operation o! ROI0, attracts RAO0I to complete a circuit extending by way of CI20 tor energizing the lower winding of R020. The relay R020, in turn, operates to interrupt, at RA02I, the above-traced circuit i'or M000. 'I'he magnet M000 now restores to step the associated switch wipers 00|, 002 and 000 one step and to interrupt, at RAO0I, the operating circuit for R020. The relay R020 again restores to recomplete the operating circuit for M000. 'I'his interrelated operation of M000 and R020 continues until such time as the wipers or the switch 000 are stepped to a position corresponding to the calling line II. In the present assumed case, wherein the control conductor C000' extends to the third contact oi' the contact set 000, the switch 000 is caused to take three steps from the position shown in the drawings, at which time the switch wiper 000 engages its associated third contact to complete a circuit for energizing the upper winding of R040. This circuit extends from battery at the cut-oi! relay R020 by way of RA004, RAOI0. C000', the switch wiper 000, the upper winding of R040, CI21, the switch wiper 010, Ci02 and through the upper winding of R020 to ground at RA000. Since the upper winding oi R020 is now being energized over a circuit which is independent of M000, this relay stays operated to hold open the operating circuit for M030. As a result. the interrelated operation of this relay and M000 is discontinued. When the upper winding of R040 is energized over the above-noted circuit, this relay partially operates to complete, at RA044, a circuit for energizing its lower winding, this latter circuit extending from ground at RA442 by way of RA400, CIIO,`RA044, and the lower winding of R040 to battery. With its lower winding energized, R040 completes its operation to interrupt, at RA040, a further point in the operating circuit for M000 and to complete, at RA040, a circuit ior energizing the cut-oil relay R020 associated with the calling line Il. At RA040, the relay R040 cornpletes a circuit for energizing M060, thereby to cause the ailotter means to operate in the manner pointed out hereinafter to a position correspending to the next idle link.

battery.

Cil

'I'he above-noted circuit for energizing the cutoi relay R320 extends from ground at RA442 by way of RA406, C||0, RA344, RA345, the switch wiperl 383, C393', RA3I5, RA304 and through the winding of R320 to battery. This relay, upon operating, short circuits, at RA326, the armatures RA3|5 and RA304 thereby to provide a holding circuit for itself and, at its armatures RA32| and RA322, interrupts the above-traced operating circuit for the line relay R3l0. The last-inentioned relay, upon restoring; removes, at RA3| I, ground from the start conductor-CI24; opens, at RA3|5, the marking path as traced above; and completes, at RA3|2 and RC3I4, an alternative path for applying ground to the control conductor C394 from the ground extended to C393. At RA322, the cut-off relay, upon operating, completes an obvious circuit for energizing the slowto-release line lockout relay R300, which latter relay now operates to open a further point in the path for applying ground to the start conductor C|24 and a further point in the path for marking the calling line in the bank contacts oi the rotary switch 380.

Immediately upon the operation of the switching relay R340, the subscriber's loop incoming from the calling substation A is extended by way of RA34| and RA342 to the link line conductors C|04 and C|08. The extension of this loop to these conductors results in the completion of an alternative operating circuit for R310, which circuit extends from ground at the lower winding of this relay by way of the said lower winding, RA364, RC356, RA355,RA342, the switch wiper 382, C393, the bridge across C393 and C932 at the substation A, C392, the switch wiper 38|, RA34I, RA352, RC353, RA362 and through the upper winding of R310 to battery. With the subscribers loop thus extended and the relays R300, R320, R340, R310, R403, R4|0 and R420 operated, the connector portion of the link is conditioned to respond to impulses transmitted thereto from the impulsing device conventionally provided at the calling substation A.

Operation of the allotter means to find an idle link Before describing the operation of the link during the transmission of impulses thereto from the calling subscriber's substation, the operation of the allotter means illustrated in Fig. `6 will be considered. In this connection, it will be noted that a busy link is identied by the presence of ground on the guard conductor corresponding to Cl 30 for the link illustrated, whereas, if the link is idle, battery is present on this conductor. Thus, immediately u pon the operation of R340, ground is applied, at RA346 and RC341, to CI30, thereby to mark the associated link as busy in the bank contacts oi"r the rotary switch 610 and, with this switch standing in the position corresponding to this link, a circuit is completed for energizing the motor magnet M660. This circuit extends from ground at RC341 by way of RA346, CI30, the switch wiper 61|, CI33, RA606, and through the winding of M660 to battery. The magnet M660, upon being energized, steps its associated wipers 61| to 614, inclusive, one step so that the above-traced operating circuit is interrupted at the wiper 61| and the bank contact terminating C|30. If the link corresponding to the second contact of the contact set -615 is busy, ground is present on this contact and a new circuit is completed for energizing M560, thereby to cause this magnet to step the enumerated wipers a second step. This action is continued until such time as the-wiper 61| encounters a contact in the associated set 615 corresponding to an idle under-connector link. When this occurs, no further circuit is completed for energizing M660, and the operation of the rotary switch 610 is interrupted. With the switch standing in this new position, circuits are prepared for seizing the selected idle finder-connector link in the exact manner just described with reference to the link illustrated in Figs. 3 to 5, inclusive.

As was pointed out above, operation of R340, following the operation of the link to seize the calling line is followed by operation of the cut-olf relay R320 and the restoration'bf the line relay R3|0. The last-mentioned relay, upon restoring, interrupts at RA3| the operating circuit for R6|0, which latter relay, upon falling back, interrupts, at RABIS, the operating circuit for R605. 'Ihe relay R605, u pon restoring, opens the only remaining operating circuit for R620, causing this relay to restore. Thus, the apparatus included in the allotter means illustrated in Fig. 6 is restored to normal and is prepared for further use.

As indicated previously, the seizure of any link is accompanied by the operation of the relay corresponding to R420. Each of these relays, upon operating, functions to interrupt, at RA42| and RC423, a point in the operating circuit for R600. Hence, when all of the links are busy so that all of the relays corresponding to and including R420 are energized, the operating circuit for R600 is completely open and this relay rstores. As a consequence, it is impossible for a calling line to initiate the operation of any finder switch in search thereof. A call initiated at a time when such a condition prevails will, therefore, not be rewarded with dial tone and cannot be extended until such time as one of the links is again idle. Obviously, when one of the links is released following an all-links busy condition, the all-links busy relay R600 is again caused to operate. From the preceding description it will be apparent that when all of the links are busy the switch 610 continues to operate in search of an idle link until R600 restores to cause the sequential restoration of R6 0 and R605. The lastmentioned relay upon restoring opens the operating circuit for M660 causing the switch 610 to discontinue its hunting operation. Thereafter, and when R600, R6|0 and R605 are again caused to operate, the switch 610 is started in operation to find the idle link responsible for the reoperation of R600 and to assign this link to the calling line responsible for the operation of R6|0 and R605.

Operation of the power and supervisory apparatus Before describing the operation of 'the oonnector portion of the link to select the called line |6 in response to the transmission of impulses thereto from the calling substation A, and in order to render this portion of the description more clearly understandable, the operation of the power and supervisory apparatus, illustrated in Fig. '1 and the lower portion of Fig. 6, will now be considered. As pointed out previously, this apparatus includes an invertor circuit, indicated as the ringing current source, for transforming the exchange battery voltage into an alternating ringing voltage, a plurality of ringing current conductors, C|54, C|55, C|56, C|51 and C|58, and a ringing interrupter comprising the rotary switch 110 and the associated control means for controlling the connection of the ringing current conductors to the alternating voltage source. 'I'he arrangement of the enumerated elements is' such that each of the ringing current conductors has impressed thereon the voltage of the source for coded intervals which are different for each of the various conductors. The power'and supervisory apparatus also includes a timing device for generating spaced supervisory impulses which are utilized in the operation of any one of the links to lockvl out a faulty line until the fault is corrected, and are also utilized in the operation of the allotter means to cause different links successively to be assigned to a callingvline in the event.the assigned links fail to ilnd a calling line. This apparatus further comprises means partially controlled by the ringing current interrupter for causing the connector portion of an operated link to search for an idle outgoing trunk line when a connection extending by way of such a line is desired by the calling subscriber. An additional feature of this apparatus pertains to the provision of circuit elements arranged to form a busy-tone current source and a dial-tone current source all in the manner described immediately hereinafter.

As pointed out above, the operation of the powerand supervisory apparatus is initiated by the operation of R126 during the sequence of operations just described and immediately the link illustrated 'is taken for use. Upon operating, R126 completes a circuit for energizing R636,

R645, R655, the upper winding of R166, and the upper windings of R165 and R161, respectively. 'I'his circuit extends from ground at RA121 to C166 where it divides, separate branches thereof extending through the designated windings of the various relays noted to battery. Through the operation of these relays, each of the various units referred to above is started in operation to perform the respective functions assigned thereto. Thus, the operation of the invertor illustrated in the lower left-hand corner of Fig. 6 is initiated upon the operation of the relays R636 and R645. The last-mentioned relay upon being energized attracts its weighted amature RA646 to prepare at RC646 a path for shortcircuiting its own winding. This path is subsequently completed upon the operation of the slow-to-operate relay R636 and extends from battery at RA646 by way of RC646, RC634, RA632 and through thewinding of RA645 back to battery. 'I'he relay R636, upon operating; also completes, at RA631, a circuit extending from ground on C136 for energizing the lower winding of R646, which latter relay operates after an inter# val, to open at RA642 and RC643, the path normally short-circuiting its upper winding and to prepare at RA641 a circuit for energizing the upper winding 662 of the transformer 666. With R645 deenergized, due to the short-circuited condition of its winding, the weighted armature RA646 is released and, upon its back stroke, engages RC641 to complete the above-noted circuit for energizing the transformer winding 662, this circuit extending from ground at the inductance coil 668 by way of the inductance coil 666, the winding 662, RA641, RC641 and RA646 to battery. The weighted armature RA646 upon its back stroke also interrupts the above-traced path short-circuiting the winding of R645, causing this winding to be energized in series with the resistor 636, whereby R645 again attracts RA646. On its second forward stroke the armature RA646 interrupts, at RC641,l the abovetraced circuit for energizing the transformer winding 662 and completes, at its associated contacts RC646 and RC646, circuits for respectively energizing the lower winding 663 of the transformer 666 and the previously-traced path for short-circuiting the winding of R646. The energizing circuit for the winding 663 extends from ground at the inductance coil 666 by way of the inductance coil 666, the winding 663, RC644,

RAMI, RCB and RA646' t0 battery. Obvlously, with thewinding of R645 short-circuited, the armature spring RAI" is again released and is again carried by its weighted end into engagement with RC641 to open the above-traced circuit for energizing the transformer winding 663 and to recomplete the above-traced circuit for energizing the transformer winding 662. The amature noted, namely RA646, continues to vibrate alternately to complete the two abovetraced circuits for energizing the two transformer windings 662 and 663. These two windings are connected in opposition and, as a result, an alternating voltage is induced in the secondary winding 661, which is utilized for ringing purposes in the manner pointed out in detail hereinafter. Obviously, the frequency of this voltage is determined by the vibrating period of the amature spring RA646.

For the purpose of preventing arcing at the contacts controlled by the relay R645, a suppressor network is connected in shunt with the two seriesconnected windings 662 and 663. 'Ihis network comprises the condensers 666 and 661 and the resistors 664 and 666 connected in series,

with the Junction point between the two resistors connected to battery.` The two inductance coils 666 and 666 are provided for the purpose of suppressing, in a conventional manner, transient currents which would interfere with the conversation ensuing over an established connection.

From the foregoing description it will be apparent that ringing voltage is generated continuously so long as R126 is maintained energized. This voltage is adapted to be impressed by a selected one of the five ringing current conductors C154l C155, C156, C151 and C156, comprising the cable 156, on one or both of the line conductors of a called line selected by any one of the links embodied in the office I6. The various paths for applying the voltage developed across the secondary windings of the transformer 666 to the five enumerated conductors are interrupted, through the operation of the rotary switch 116 and the control relays R161, R116 and R115, in a manner such that the ringing voltage as ultimately applied to the conductors of a called line is coded in a particular manner. More specifically, the arrangement is such that coded ringing voltage representing one long for each ringing cycle is applied to the ringing current conductor C156; coded ringing voltage representing two shorts for each ringing cycle is applied to C166; coded ringing voltage representing three shorts is applied during each ringing cycle to C154; coded ringing voltage representing four shorts is applied during each ringing cycle to C151: and coded ringing voltage representing one long and one short is applied during each ringing cycle to C155. 'I'he 'operation of the elements noted to interrupt the ringing voltage so that codes of the designated characteristics are applied to the respective enumerated ringing current conductors is initiated. as was pointed out above, upon the operation of R126 to complete a. circuit for energizing R161. It will be noted that with the three relays R161, R116 and R115 deenergized, the windings of R116 and R115 are short-circuited over obvious paths.

When, however, R101 operates, it opens at RA108 a point in the path short-circuiting the winding of R1I0, permitting the last-mentioned relay t0 operate and interrupt at RATII the path normally short-circuiting the winding of R1 I5. The relay R1 I5 in turn operates to complete, at RA1 I 6, an obvious path for short-circuiting the winding of R101, which latter relay now restores to recomplete at RA108 the path for short-circuiting the winding of R1 I0. When R1I0 falls back, it in turn recompletes at RA1|| the path for shortcircuiting the winding R1| 5, and the last-mentioned relay, upon restoring, opens, at RAlIB, a point in the path short-circuiting R101. Thus. the three relays noted, namely R101, R1I0 and R1|5 operate sequentially and continuously so long as R is held operated.

Each time R1I0 operates it not only functions to open the path for short-circulting the winding of R1|5, but, in addition, completes a circuit for energizing M embodied in the rotary switch 110. This circuit extends from ground at RA12| by way of CI36, RA1|2, CI62 and through the winding of M165 to battery. Thus M165 is caused to drive its wipers 11| to 115, inclusive, at the rate of one step for each complete operating cycle of the three relays R101, R1I0 and R1|5.

At any given time when the two relays R1I0 and R1| 5 are simultaneously in an energized condition, paths are prepared for extending the voltage developed across the secondary windings of the transformer 680 to each of the iive enumerated ringing current conductors. Thus, with these two relays in their respective operated positions CI38, which extends to the ungrounded terminal of the transformer winding 68|, is connected by way of RC1I8, RA1I1, RA1|4 and C|63 to the common conductor C|6| which extends to selected ones of the various contacts in the three contact sets 19| to 193. By a proper selection and interconnection of the contacts to C|6|, ringing voltages, interrupted in accordance with the characteristics mentioned above, are applied to the respective ringing current conductors. For example, the conductor C|58 is terminated atthe wiper 113 associated with the contact set 193, and the common conductor C|6| is connected to each of the second, third, eighth, ninth, fourteenth, fifteenth, twentieth and twenty-rst contacts of this set. With the contacts of the set 193 thus connected, the operation of the switch is characterized by the application of voltage representing two shorts to the conductor CI58.

Assuming the switch 110 is standing in the position shown, when the cyclic operation of R101, R1I0 and R1|5 is started, the above-traced circuit for M165 is completed upon the initial operation of R1 I0. As a result, the wipers 11| to 115 are stepped to their respective first olf-normal positions so that the wiper 113 .engages its associated second contact. Shortly thereafter, R1|5 operates to complete a path for applying ringing voltage to CI58, this path extending from CI38 by way of RC1|8, RA1I1, RA1I4, CIGS, CIBI, the second contact of the set 193 and the switch wiper 113 to CI58. Since the common conductor CI 6I is also connected to the respective second contacts of the sets 19| and 192, this voltage is also applied by way of the switch wipers 11| and 112, respectively, to the ringing current conductors CI51 and CI54. Shortly thereafter, and when R101 operates, R1I0 restores to interrupt, at RA1I4, a point in the above-traced path for applying the voltage to CIGI. Thereafter, and when R1I0 again operates to cause M165 to step its associated wipers a second step or into engagement with their respective associated third contacts and R1|5 again operates, the paths noted are again completed, since the second and third contacts of each of the contact sets 19| to 193, inclusive, are connected together. When, however, the wipers of the switch 110 are stepped to their respective third oE-normal positions, the above-'traced path for applying ringing voltage to CI58 is broken at the wiper 113, since C|6I is not connected to the fourth, fth, sixth and seventh contacts of this contact set. Hence only two short pulses of ringing voltage are applied to C|58 during the period when the wiper 113 is traversing the iirst six active contacts. In the case of the contact set 192, however, it will be noted that the conductor C|6| is connectedto the second, third and fourth contacts so that three `short pulses of ringing voltage are applied to C|54 during movement of the switch wipers over the first six active points. In the case of the contact set 19|, wherein C|6| is connected to the second, third, fourth and fifth contacts, four short pulses are applied to the associated ringing current conductor CI51. It is noted here that movement of the switch wipers 11| to 115, inclusive, over six points represents a complete ringing cycle. its sixth to its seventh off-normal position, a new cycle is started, since the eighth and ninth contacts of the set 193 are connected to CI6I in the manner previously described. In a similar manner, when the wipers 112 and 11| are stepped from their respective .sixth to their respective seventh olf-normal positions, a new ringing cycle is initiated. It will be apparent, therefore, that six complete operating cycles for the relays R101, R1I0 and R1I5 are required to produce a complete ringing cycle.

The method by which ringing voltage is applied to C|56 for a relatively long time interval during each ringing cycle thereby to provide a ringing pulse representing one long, is slightly different from that just described for the production of pulses representing shorts. The production of long pulses is under the control of R180, having an operating circuit controlled by the wiper 114 in cooperation with its associated contacts. Thus. during the various ringing cycles, the wiper noted completes circuits extending by way of the second, third, eighth, ninth, fourteenth, fifteenth, twentieth and twenty-first contacts of the set 194 and C|60 for energizing the slow-to-release relay R100. This relay, upon operating, completes at RA184 and RC186, a path extending from C |38 for applying to C|56 the voltage developed across the secondary winding 68|. Since the conductor C|60 is multipled between the various enumerated contacts of the contact set 194, it will be understood that R is caused to operate once during each ringing cycle and is held operated for the time interval required for the wiper 114 to traverse two of its associated contacts. Due to the prolonged portion of each ringing cycle when R180 is energized, a voltage pulse` is applied to C|56 during each ringing cycle which is of sufllcient length to distinguish the same from the short pulses applied to CI54, CI 51 and CI58.

With respect to the coded ringing voltage applied to C|65 and comprising one long impulse and one short impulse for each ringing cycle, it will be noted that the long impulse is attained through the operation of R180 directly to apply at RA18| and RC183 the voltage on CI38 to the ringing current conductor CI55. This long pulse is held until the wiper 114 is stepped from engage- Thus, when the wiper 113 is stepped from ment with its associated third contact and into engagement with its associated fourth contact, at which time the operating circuit for R is opened and an alternative path is prepared, at the switch wiper 112 and its associated fourth contact, for applying a short pulse of voltage to CI55. This path is completed upon the operation of R1l5 and extends from C|00 by way of RC1I0, RA1I1, RA1I4, Cl00, CISI, the fourth contact of the. set 102, the switch wiper 112, RC102 and RA10I to CI55. It is believed clear without further discussion that the production of this particular code occurs once during each ringing cycle.

Ground potential is, at all times, applied to each of the ilve enumerated ringing current conductors for the purpose of energizing the ring cut-oil' relays of the links in the manner described hereinafter. Thus, those contacts of each of the contact sets 10| to 103, inclusive, excepting the iirst contacts of each set, which are not active in the paths for applying voltage to the ringing current conductors, are grounded so that ground is directly applied to the respective conductors CI50. CIM and C|51 when the respective associated wipers are stepped into engagement therewith. For the same purpose, the armatures RA1I0 and RA1I1, embodied respectively in the` relays R1|0 and R1I5, are arranged to be oonnected to ground, when restored, so that ground may be applied to the active contacts of the contact sets 19| to 193, inclusive, for a portion of each operating cycle of the relays R101, R1 i 0 and R1 I 5. In a similar manner, the armature RA100 embodied in the relay R100 is arranged to be connected to ground at RC during those intervals when R100 is restored, 'so that ground is extended by way of the elements noted to C|50. Direct current ground potential is also applied to each of the iive enumerated conductors during the periods when alternating ringing current voltage is applied thereto, it being extended. during such periods, from the grounded terminal of the transformer winding 00| and by way of the conductor C|00 to each of these conductors.

Referring now more particularly to the operation of the relay R100, it is pointed out that this relay functions as a source of busy-tone current. Thus, when the upper winding of this relay is energized from ground at RA12I, upon operation of R120, it operates to complete at RA10I a circuit also extending from ground at RA12i for energizing its lower winding. The two windings of R100 are connected in opposition so that when the lower winding thereof is energized, the relay immediately restores to open at RA10| the circuit for energizing its lower winding. Immediately this circuit is opened, the upper winding is eil'ective to cause the relay to reoperate. Hence, it will be seen that this relay operates in the inanner of a buzzer. Each time R100 operates, it short-circuits, at RA10I, the circuit comprising the resistor 103 connected in series with the exchange battery. Due to the repeated short-circuiting of these two series connected elements, an 'alternating voltage is developed across the resistor 103, which is extended by way of the filter choke 100 and the armature RA1|3 to the busytone conductor C I I1. From the above description it will be` apparent that the cyclic operation of R1|5 causes an interrupted busy-tone voltage to be applied to the busy-tone conductor noted.

The relay R105 operates in a manner similar to that just discussed with reference to R100 to develop across the resistor 102 an alternating voltage which comprises a dial tone current source. Thus, when R120 operates, it completes, at RA12I, an obvious circuit for energizing the upper winding of R105. 'Ihis latter relay immediately operates to complete, at RA100 and RA101, a circuit for energizing its lower winding. 'Ihe two windings of R105, -like those of R100, are connected in opposition so'that, when the abovenotedxcircuit is completed, this relay restores to open the operating circuit for its lower winding. Immediately this occurs, the upper winding is again effective and the relay reoperates. It will be seen, therefore, that R105 functions in the manner of a buzzer so long as R120 is held operated. Each time R105 operates, it short-circuits, at RA100 and RA101, the sexies circuit comprising the exchange battery and the resistor 102. Hence, through the repeated operation of R105, an alternating voltage is developed across the resistor 102 which is impressed through the illter choke 104 on the dial tone current conductor CI5I. This current is generated so long as the relay R120 is maintained in its operated position.

As noted previously,'the power and supervisory apparatus includes means for controlling the connector portion of any seized link in its operation to select an idle trunk outgoing from the exchange I0 and following the dialing of the number identifying the group of trunk lines. Speciilcally, this means comprises the two relays R050 and R055, which function to apply ground pulses to the several supervisory conductors C100, CI00', etc. extending, respectively, to the control apparatus individually embodied in the several links provided in the exchange I0. 'I'he operation of these two relays to perform the function noted is initiated when R120 operates to complete, at RA12I, a circuit extending by way of CI00 for energizing the slow-to-release, slow-to-operate relay R555. This relay, upon being energized,

RA656 its operating winding and to complete at RA051 an obvious circuit for energizing R050. The last-mentioned relay now operates to complete, at its armatures' RA05I and RA052, etc.,

paths for applying ground to the supervisory conductors CHQ, CI30, etc. After the winding of R655 has been short-circuited for an interval, this relay restores, so that its winding is again energized to cause its reoperation. During the period of restoration, however, R550 is caused to restore, so that ground is removed from the various supervisory conductors noted. It will be appreciated that the repeated operation and restoration of R055 to cause the repeated operation and restoration of R050 continues so long as ground is maintained on CISG, or, so long as R120 is held operated.

'I'he timing device, referred to brieily above, comprises more particularly the plurality of interrelated relays R124, R130, R135 and R140, which relays are controlled by means including the rotary switch 110. This device functions to apply ground pulses to the supervisory leads CIN,

lCI and CI35 at spaced time intervals, which ground pulses are utilized to cause the allotter means illustrated in Fig. 6 to assign a new link in search of a calling line in the event a first assigned link fails to nd the calling line, and further to cause a defective line to be locked out of service. The particular portion of the switch which enters into the operation of the timing device comprises the contacts of the set 134 and the associated wiper 110. Thus when R operates to initiate the operation of the switch 110 operates after an interval to short-circuit at im and when this switch steps its wipers to their respective third oir-normal positions, a circuit is completed at the switch wiper 114 for -energizing R135, this circuit extending by way of the wiper noted, the fourth contact of the set 194, CIBG, RC146, RA144, and through the winding of R135 to battery. The relay R135, upon operating, completes, at RA136, a path for applying ground to the supervisory conductor or time pulse lead CI31. This path extends from ground on CI22, RA129, RA14I, RC142 and RA136 to CI31. The Vcompletion of this path in the manner noted represents the first ground pulse applied to thev supervisory conductor CI31. The relay R135, upon operating, also prepares, at RA138, a locking circuit forA itself, but this locking circuit is not rcompleted at this time due to the deenerglzed condition of R140. When the wipers of the switch are stepped from their respective third off-normal positions, the above-traced operating circuit for R135 is broken at the switch wiper 1'l4,causing this relay to restore.

Thereafter, and when the switch wipers are rotated into engagement with their respective associated twenty-fourth contacts, a circuit is completed for energizing R140, this circuit extending from ground on the switch wiper 114 by wayof the twenty-fourth contact of the set 194, CI68 and through the lower winding of R140 to battery. The relay R140 now operates to complete at RA141 a locking circuit for itself, which circuit includes the upper winding of R140 and extends from ground on CI22. At RA14I, the relay R140 opens a point in the above-traced path for applying ground to CI31 and prepares a point in the holding circuit for R135. At RA144 and RC146, the relay R140 opens a point in the above-traced operating circuit for R135 and prepares an alternative operating circuit therefor.

During continued operation of the switch 110 the wiper 114 is brought into engagement with the twenty-third contact of the contact set 194 so that the last-mentioned operating circuit for R135 is completed, this circuit extending from ground on the wiper 114 by way of CI61, RC145, RA144 and through the winding f R135 to battery. Upon operating the second time. R135 completes at RA138 a holding circuit for itself, which circuit extends from ground on CI22 by way of RA128, RA14I, RC143, RA138 and through the winding of R135 to battery. At RA 136, the relay R135 prepares a point in the above-traced path for applying ground to CI31, this path now being open at RA14I and RC142 of the operated relay R140. At RA131, the relay R135 opens a point in the holding circuit traced hereinafter for R124 and at RA139 it prepares a point in the operating circuit for R130.

After a time interval during which the switch 110 continues to operate, the switch wiper 114 is rotated into engagement with the twenty-second contact of the contact set 194, thereby to complete a circuit extending by way of CI65 and RA139 for energizing R130. The last-mentioned relay, upon operating, completes, at RA13I, a holding circuit for itself and prepares, at RA132, a circuit for energizing R124. The holding circuit noted, namely. that forR130, extends from ground on CI22 by way of RA129, RA13I and through the winding of R to battery.

After a still further time interval during which the switch 110 continues vto operate, the wiper 114 is driven into engagement with the nineteenth contact of the set 194, thereby to complete the above-noted operating circuit for R124, which circuit extends from ground on the wiper114 by way of CI64, RA132, and through the winding of R124 to battery. 'I'he relay R124, upon operating, opens at RA120 a point in each of the abovetraced holding circuits for the relays R130, R and R140, causing these relays to restore. At RA129, the relay R124 prepares a holding circuit for itself, which circuit is completed at RA131 before the slow-to-release relay R130 falls back to restore its armature RA132. In this connection it is pointed out that the switch 114 stands in engagement with its associated nineteenth contact for a time interval sumcient to permit each of the relays R130, R135, and R to restore, and the relay R124 to be energized.

It will be noted that with R124 deenergized, ground is normally applied from RA125 by way of RC126 to the supervisory conductor CI49. When, however, R124 operates, ground is removed from this conductor and is applied to the supervisory conductor CI35 at RC121. The operation of this armature to remove ground from CI48 and to apply ground to CI35 is responsible for certain of the supervisory operations referred to more specically in a following portion of the specication.

The relay R124 is held operated until such time as the switch wiper 114 is again driven into engagement with the fourth contact of the set 194, at which time the previously traced operating circuit for R135 is completed. The relay RA135, upon operating a third time, opens at RA131 the above-traced holding circuit for RA124, causing this relay immediately to restore. At RA136 the relay RA135 prepares a point in the path described above for applying ground to the supervisory conductor CI31, this path being subsequently completed when R124 restores its armature RA128. Thus, a second ground pulse is applied to C I 31 a denite time interval following the application of the rst ground pulse to this conductor. In view of the preceding description, it will be understood that the cyclic mode of operation of the four relays R124, F130, R135 and R140 under the control of the switch 110 is continued so long as R120 is held operated to maintain the switch 110 in operation.

Asnoted above, rotation of the switch wipers embodied in the switch 110 over twenty-five points corresponds to the completion of four ringing cycles. In order that this switch shall be conditioned to start a ringing cycle irrespective of when the power and supervisory apparatus is released, there is provided an alternative circuit for maintaining R120 energized independently of the condition of R420 embodied in the control apparatus of the link illustrated, or a corresponding relay of another link. This circuit extends from ground at the switch wiper 115 by way of CI59, RA122 and through the winding of R120 to battery. As shown, the conductor CI59 is connected to each of the contacts of the contact set corresponding to the active points of each of the other contact sets. Hence, if R420 is caused to restore to remove ground from the start conductor CI22 while the wipers of the switch 110 are standing in their respective fourth off-normal positions, for example, the relay R120 is held energized over the circuit just described until the switch wipers are rotated to their respective sixth olf-normal positions, at which time this alternative circuit for R120 is interrupted, causing an interruption in the cyclic operation of the control relays R101, R'II 0 and R1 I 5. There results the stopping of the switchI 110 with the wipers thereof standing in 

